Interrupt remapping table entries for MSI interrupts set up by bridge devices did not get any source validation set up on them, allowing misbehaving or malicious guests to inject interrupts into the domain owning the bridges.
In a typical Xen system bridge devices are owned by domain 0, leaving it vulnerable to such an attack. Such a DoS is likely to have an impact on other guests running in the system.
A malicious domain, given access to a device which bus mastering capable, can mount a denial of service attack affecting the whole system.
Acknowledgements:
Red Hat would like to thank the Xen project for reporting this issue.

Statement:
Not vulnerable.
This issue did not affect the versions of the kernel-xen package as shipped with Red Hat Enterprise Linux 5.
This issue did not affect Red Hat Enterprise Linux 6 and Red Hat Enterprise MRG as we did not have support for Xen hypervisor.